The overall aims of the project are to delineate adrenergic receptor subtypes in neural tissue by radioligand binding techniques, and to compare the binding and pharmacologic characteristics, receptor-effector coupling, and regulation, alpha1-, alpha2- and beta-adrenergic receptors in neural and non-neural tissue. Particular emphasis is placed on the alpha2-receptor which is inversely coupled to adenylate cyclase. In the past year, alpha2-receptors in neural (brain and neuroblastoma x glioma NG 108-15 hybrid clonal cultured cells) and non-neural (platelets, cultured human fibroblasts and aorta) tissue have been labeled with 3H-yohimbine (antagonist), 3H-epinephrine (full agonist) and 3H-p-aminoclonidine (partial agonist). In each tissue, multiple affinity states of the receptor have been observed, and correlated with the cellular response of inhibition of adenylate cyclase activity in NG 108-15 cells and platelets. Agonist-induced desensitization has been examined for NG 108-15 alpha2-receptors, and appears to occur in sequential stages of uncoupling and receptor loss. Regulation of rat cortex alpha2-receptors has also been examined after (a) chronic brain isoproterenol infusion, (b) noradrenergic denervation, and (c) congenital noradrenergic hyperinnervation. Current studies are designed to characterize alpha2-receptors using more specific antagonist radioligands; compare -receptor desensitization, and investigate the existence of spare receptors, in neural and non-neural tissues; label alpha2-receptors in intact tissue; examine alpha2-mediated inhibition of adenylate cyclase in brain tissue. Alpha2-Adrenergic receptors are believed to play an important role in hypertension, depression, addiction, and antidepressent therapy. The studies in this project are helping to elucidate the biochemical mechanisms of, and the significance of neuronal connexions in, alpha2-receptor regulation.